Natural gas has been used as a fuel for piston engine driven vehicles for over fifty years but the drive to improve efficiency and reduce pollution is causing continual change and improvements in the available technology. In the past, natural gas driven vehicles (NGV) were naturally fumigated, that is, natural gas was introduced into the cylinders through the intake manifold, mixed with the intake air and fed into the cylinders at relatively low pressure. The fuel supply system for such an NGV is relatively simple. Fuel is held in and supplied from a liquified natural gas (LNG) vehicle tank with working pressure just above the engine inlet pressure, or from compressed natural gas cylinders (CNG) through regulators which reduce the pressure to the engine inlet pressure.
Compressed natural gas (CNG) is commonly stored at ambient temperatures at pressures up to 3600 psi (24,925 kPa), and is unsuitable for trucks and buses due to the limited operating range and heavy weight of the CNG storage tanks.
On the other hand, liquified natural gas (LNG) is normally stored at temperatures of between about -240.degree. F. and -200.degree. F. (about 150.degree. C. and -130.degree. C.) and at pressures of between about 15 and 100 psig (204 and 790 kPa) in a cryogenic tank, providing an energy density of about four times that of CNG.
However, better efficiency and emissions can be achieved if the natural gas is injected directly into the cylinders under high pressure at the end of the compression stroke of the piston. This requires a fuel supply system which can deliver the natural gas at a pressure of 3000 psig and above. This makes it impossible to deliver the fuel directly from a conventional LNG vehicle tank and it is impractical and uneconomical to build an LNG tank with such a high operating pressure. Equally, it is impossible to deliver the natural gas fuel directly from a conventional CNG tank as the pressure in such a tank is lower than the injection pressure as soon as a small amount of fuel has been withdrawn from the CNG tank. In both cases, a booster pump is required to boost the pressure from storage pressure to injection pressure.